This invention relates to methods of depositing films of a silicon substance, such as silicon nitride and polycrystalline silicon, on integrated circuit wafers; and more particularly, it relates to methods of mass producing such films in a quartz chamber with improved structural uniformity and reduced defects.
As is well known, an integrated circuit is made by depositing and patterning films of several different materials on top of each other in layers on a silicon wafer. One of these films is often made of silicon nitride. It is used to insulate one conducting layer from another. Also, it is used as a mask for an ion implant step and as a mask for a selective oxidation step.
Another common film of material in an integrated circuit is made of polycrystalline silicon. It is generally used as a conductor; and it is made conductive by being doped with phosphorus. Doped polycrystalline silicon is used, for example, as a gate in field effect transistors.
However, if microscopic imperfections occur in the structure of such films during their fabrication, then the integrated circuit into which they are incorporated may be rendered inoperable. For example, a small pinhole in a silicon nitride film that is used as an insulator between two conductors can result in a short between those conductors.
Likewise, a small pinhole in a silicon nitride film that is used as a mask for an ion implant step or an oxidation step can respectively result in an unintended portion of the circuit being implanted or oxidized through the pinhole. Structural imperfections in polycrystalline silicon films also cause integrated circuit malfunctions.
When films of silicon nitride or polycrystalline silicon are deposited on integrated circuit wafers in a deposition chamber, it is known that the same silicon substance is also deposited on the internal walls of the chamber. Thus in a mass production environment where multiple batches of wafers are processed one after the other in the same chamber, the silicon substance on the internal walls builds up. This film must be periodically removed from the chamber walls before it gets too thick; otherwise it will flake off and cause microscopic structural imperfections in the film that is being deposited on the wafers.
In the semiconductor industry, the chambers in which the silicon nitride and polycrystalline silicon films are deposited are almost always made of quartz. Such quartz chemical vapor deposition chambers are available from National Quartz Corporation of Orange, California and Quartz International Corporation of Santa Clara, California. One reason for using quartz is that silicon nitride and polycrystalline silicon films adhere quite well to quartz; and thus the number of batches of wafers that can be processed before the depositin chamber needs a cleaning is maximized.
In the past, it has been a standard practice in the semiconductor industry to remove films of silicon nitride and polycrystalline silicon from the internal walls of the quartz deposition chamber by means of an acid etch. However, as the present invention teaches, such a removal step has some serious side effects that prevent the number of defects in the deposited films from being as small as the present invention achieves.
Accordingly, a primary object of the invention is to provide a method of mass producing films of silicon nitride and polycrystalline silicon with greatly reduced numbers of structural defects.